The present invention relates to an optical information recording medium of a phase-change type which can record, reproduce, erase, and rewrite information using a laser beam.
A certain kind of alloy consisting of Te (tellurium) as a base material has been known as a material which causes a reversible phase transition. For example, in the U.S. Pat. No. 3,530,441, S. R. Ovsinsky et al have disclosed for the first time that a thin film such as Te.sub.85 Ge.sub.15, Te.sub.81 Ge.sub.15 S.sub.2 Sb.sub.2, or the like causes a reversible phase transition in accordance with the irradiation of a high density energy of a laser beam or the like. Since then, an optical data recording medium in which an absorptive layer is formed by a material which causes a reversible phase transition is being developed.
On the other hand, in an optical information recording medium such as an optical disk or the like, there is a tendency such that the absorptive layer is made thin. A purpose of this tendency is to mainly reduce the heat capacity of the light irradiating portion and thereby to decrease an energy necessary for the recording and reproduction (i.e., to raise the sensitivity). At the same time, by effectively use the interference effect of the light, a change amount of the reflected light or the transmitted light before and after the recording of the optical information is increased, thereby obtaining a large signal (raising the S/N (signal-to-noise) ratio). It will be obviously understood that if the absorptive layer is merely made thin, the light absorbing efficiency in the absorptive layer decreases, so that the sensitivity deteriorates. Therefore, for example, a measure to improve the light absorbing efficiency in the absorptive layer is taken on the basis of the calculation due to the Matrix method or the like disclosed in O. S. Heavens, "Optical Properties of Thin Solid Films", page 69, issued by Dover Publications Inc., New Hork, in 1965. Practically speaking, for example, in the specification of U.K. Patent Publication No. 2079031, there has been disclosed a tri-layer optical information recording medium of the phase-change type such that a spacer layer is interposed between an absorptive layer and a reflective layer. The spacer layer functions to inhibit that an irreversible change occurs in the absorptive layer irradiated by a laser beam for recording or erasing. By forming the spacer layer, the operating life for repetitive operations of recording and erasing (i.e., a possible maximum number of times of repeating operations) is remarkably improved. The optimum values of the thicknesses of the spacer layer and absorptive layer are calculated by use of the foregoing Matrix method. The absorptive layer consists of a thin film of Chalcogenide alloy(s). The reflective layer consists of thin film of a metal such as Au, Al, or the like having a large optical reflectivity.
However, as the result of the study by the applicant of the present invention, it has been found that in the tri-layer optical information recording medium, when a rewritable optical disk was constituted by forming the absorptive layer consisting of a thin film of Chalcogenide alloy(s) and the reflective layer consisting of a thin film of Au and Al, in spite of the fact that the light absorbing efficiency in the absorptive layer is sufficiently high, the expected high sensitivity could not be obtained.
The reversible phase transition between the crystalline phase and the amorphous phase which appears in Chalcogenide alloy(s) or the like is caused by the thermal processes due to the light irradiation. Among these processes, in the process to crystallize the amorphous phase, it is necessary to keep a temperature above the crystallization temperature for a little while. However, in the case of the foregoing constitution, the heat generated by the light irradiation is mainly propagated through the reflective layer and can be easily diffused to the circumference. Consequently, the energy loss is large and the high sensitivity cannot be derived. Therefore, it is considered that if the reflective layer is formed by a thin film of a metal whose thermal diffusivity is lower than that of Au or Al, an optical information recording medium of the high sensitivity can be obtained.
In the specification of U.S. Pat. No. 4,578,684, there has been disclosed a tri-layer optical information recording medium using a thin film of simple body gold (Au), aluminum (Al), nickel (Ni), and chromium (Cr) as a reflective layer. However, as described in detail hereinbelow in the specification, even if the reflective layer was formed by the thin film of Ni and Cr, the sufficient sensitivity cannot be obtained. Namely, hitherto, in the foregoing tri-layer optical information recording medium, the advantage with the structure is not sufficiently effected and the drawback is emphasized.